My seatpost is too short

I took my shiny new bike out today and have now realised that I ordered the wrong size and it's a tad small with the seatpost up to the max mark. I'm hoping that I can change the seatpost to a longer one.

I can see from other posts that I need to be sure to get the right diameter. I presume that I should measure the apperture where the seatpost goes? If so, this seems to be 31mm.

Also am I right in thinking that the length is measured from the bit which connects to the saddle to the end? At the moment this is 300mm.

The current seatpost is a Bontrager Sport. My bike is a Trek 4500. Does anyone have any suggestions on a suitable replacement seatpost. I can see from a look at some online retailers, that you can spend loads of money on these (and probably any other bike parts!), but what sort of price should I be looking at to get one which would be good for my bike.

What you could do is go back to the bike shop and plead with them to let you swap it for a bigger size, as having the wrong size bike is a bit of a ball-ache to be honest. You could go along, explain the situation, and offer/suggest to pay them a re-stocking fee for them to be able to then resell it as an 'ex-demo' bike if they are umming and ahhing. Guage the tone - if they appear to be bending over backwards, let them swap it. Get the sob story ready, and aim for the organ player, not the monkey grinder.

If you don't want to / can't do that, then yes you can probably get a longer seat post. They should specify how long they are.

Also, the diameter will be either 30.9mm or 31.6mm - the two aren't interchangeable.

The old seat post may have it stamped on, look at it very carefully (there may/should be grease on it so you might have to give it a wipe to be able to see it.) if it doesn't, you're going to have to get vernier calipers

or if you havent got any take it along to the bike shop and get them to measure it and get you a longer one of the same diameter.

Make sure you insert it at least to the minimum insertion point, this is usually so there's at least about 100mm of it inside the frame.

Maybe you could make the most of both of Bonj's lines of approach: even if you do have a set of Vernier calipers - as which right-thinking person doesn't?!? - you could still take the post back to the shop for them to measure.
Play it by ear, plead ignorance, penury and anything else which might elicit pity, and see what's the best deal they 'll come up with.

Maybe you could make the most of both of Bonj's lines of approach: even if you do have a set of Vernier calipers - as which right-thinking person doesn't?!? - you could still take the post back to the shop for them to measure.
Play it by ear, plead ignorance, penury and anything else which might elicit pity, and see what's the best deal they 'll come up with.

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only problem with that is, once you start getting them to measure the post, you've already sort of half said you're happy with the bike. If you go in there with the attitude that you want the next size up from the start, you're more likely to walk away with that. If it's too small with the seatpost all the way up then the frame IS too small, definitely - there's no two ways of putting it.
If you get them thinking all they have to do is sell you another seatpost then they'll latch on to that as an easy option.

I'm afraid my bike was ordered over the 'phone as it was an insurance replacement, so no LBS to go back to.

I couldn't see a size written on the actual post, but there was a lot of grease. It's getting a bit dark now, so I'll pop out to the garage in the morning and see if I can see the size written on it when I've wipped off the grease.